The geometric structure of liquid water has been investigated in detail by many techniques, but many details are still under debate, such as the actual number of hydrogen bonds (at a given time) between the various water molecules. Even less is known about the electronic structure. Since it is the intermittent bonding between water molecules that gives liquid water its peculiar characteristics, the electronic structure plays a crucial role in understanding the properties of the liquid state. Consequently, information essential for insight into chemical and biological processes in aqueous environments is lacking. To address this need, researchers from Germany and the U.S. have used soft x-ray spectroscopy at the ALS to gain detailed insight into the electronic structure of liquid water. Their spectra show a strong isotope and a weak temperature effect, and, for the first time, a splitting of the primary emission line in x-ray emission spectra. By making use of the internal "femtosecond clock" of the core-hole lifetime, a detailed picture of the electronic structure can be painted that involves fast dissociation processes of the probed water molecules.

How Do Water Molecules Talk to Each Other?

Water is the single most important substance of life. So at first sight, one would think that it is also the best-known "chemical" in the world. This is probably true, but not on an atomic scale! Surprisingly, our microscopic knowledge of water is rather limited. How do water molecules interact with each other to form the liquid structure? How many nearby water molecules is a specific molecule interacting with? How can we find out? In recent years, soft x-ray spectroscopy has emerged as a new tool to shed light on questions like this.

As in every experiment, probing a system with soft x rays also involves disturbing the system to some degree. In the present case, the disturbance is the addition of energy from the x-rays. A team of researchers from Germany and the U.S. has used this "disturbance" as a tool: by collecting high-resolution soft x-ray emission spectra, they studied the interactions between soft x-rays and liquid water. This information, in turn, was used to learn more about the interactions among water molecules themselves and the influence of temperature and isotope substitution (replacing the hydrogen with deuterium to make "heavy" water). Their findings shed new light on the unique microscopic and macroscopic properties of liquid water.

Flow-through cell for soft x-ray spectroscopy of liquids. The liquid sample enters and exits via the fast valves, flows through the channels in the Teflon® body, and passes by the silicon nitride (SiN) window through which exciting and emitted x-rays are transmitted.

In the past, the investigation of liquids has been mostly limited to experiments focusing on structural information. In contrast, the study of the electronic structure of liquids is a technical challenge that only began to be met in 2002 with the first publications about high-resolution x-ray spectroscopy of water. Recently, the German-American team used soft x-ray emission (XES) and absorption spectroscopy (XAS) to successfully probe the electronic structure of liquids. These element-specific techniques probe occupied and unoccupied electronic states, respectively, and thereby provide deep insight into the local chemical environment of the selected atomic species. The experiments were performed at ALS Beamline 8.0.1 using a custom-designed liquid-flow-through cell with a 100-nm-thick silicon nitride membrane.

The team found that their high-resolution x-ray absorption and emission spectra of normal water (H2O) and deuterated water (D2O) exhibited a strong isotope effect (i.e., a variation between the H2O and D2O spectra). Furthermore, the XES spectra of both showed a splitting of the 1b1 emission line, a weak temperature effect, and a pronounced dependence on the excitation energy. The latter can be best seen in a novel resonant-XES (RIXS) map representation.

Left: Schematic electronic structure of water, showing absorption that excites an electron to an unoccupied state and emission of an electron from an occupied state. Right: x-ray absorption (XAS, top) shows structure due to excitation to unoccupied states, and x-ray emission (XES, bottom) shows peaks due to emission from occupied states. The splitting in the 1b1 peak is clearly visible. Absorption is measured by the total-fluorescence-yield (TFY, emission intensity integrated over all energies).

The XES spectra can be well described as a superposition of two independent components, but an explanation based solely on the existence of two distinctly different hydrogen-bond configurations in the water is contradicted by the direction and the strength of the observed isotope effect. Instead, comparison of the XES spectra of liquid H2O and D2O, gas-phase water, ice, and aqueous NaOH and NaOD solutions suggests that an ultrafast dissociation of the water molecules is induced by the x-ray excitation and plays an important role for the spectral shape. In this interpretation, the high-energy component of 1b1 is ascribed to intact water molecules, while the low-energy component is related to dissociated water molecules (i.e., one proton is removed). The temperature-dependence is interpreted such that local hydrogen bonds promote the dissociation process. If the temperature is increased, more hydrogen bonds are broken, and consequently the team observed that the fraction of water molecules dissociated by the x-ray beam decreased with increasing temperature.

Left: Resonant-XES map of the electronic structure of liquid water (H2O) measured with a custom-built soft x-ray spectrometer. The horizontal axis represents the emission energy; the vertical axis shows the excitation energy; the intensity scale is color-coded. The emission spectrum at fixed excitation energy (550 eV) is shown at top (red) and the right (blue) line represents the fluorescence yield absorption spectrum. Right: Selected XES spectra of liquid water measured with the permanently installed soft x-ray fluorescence (SXF) spectrometer at Beamline 8.0.1, showing the isotope (top) and temperature (bottom) effects.

Thus, additional pieces can be contributed to the liquid-water puzzle: dynamics play an important role in understanding not only the ground-state properties but also the interaction of water with soft x rays. Comparing normal and deuterated water allows us to gain insight into the femtosecond dissociation dynamics of water molecules in a liquid environment, where the involvement of a specific H (or D) atom in a bond promotes the dissociation of the corresponding water molecule. Further experiments will be needed to fully unravel the local geometric and electronic structure of hydrogen-bonded water networks.